Spongy all-in-liquid materials by in-situ formation of emulsions at oil-water interfaces

Printing a structured network of functionalized droplets in a liquid medium enables engineering collectives of living cells for functional purposes and promises enormous applications in processes ranging from energy storage to tissue engineering. Current approaches are limited to drop-by-drop printing or face limitations in reproducing the sophisticated internal features of a structured material and its interactions with the surrounding media. Here, we report a simple approach for creating stable liquid filaments of silica nanoparticle dispersions and use them as inks to print all-in-liquid materials that consist of a network of droplets. Silica nanoparticles stabilize liquid filaments at Weber numbers two orders of magnitude smaller than previously reported in liquid-liquid systems by rapidly producing a concentrated emulsion zone at the oil-water interface. We experimentally demonstrate the printed aqueous phase is emulsified in-situ; consequently, a 3D structure is achieved with flexible walls consisting of layered emulsions. The tube-like printed features have a spongy texture resembling miniaturized versions of “tube sponges” found in the oceans. A scaling analysis based on the interplay between hydrodynamics and emulsification kinetics reveals that filaments are formed when emulsions are generated and remain at the interface during the printing period. Stabilized filaments are utilized for printing liquid-based fluidic channels.

The submitted manuscript reports on liquid-in-liquid printing of filaments that are stabilized by emulsions that form when an aqueous solution containing silica nanoparticles come in contact with an oil phase containing Span 80 micelles. Four different flow regime morphologies are reported and include single droplets, bead-on-a-string, column, and connected. The transition from single droplets to the three other morphologies is a result of the addition of silica nanoparticles and increased injection speed. Optical and scanning electron microscopy images indicate that the emulsion morphologies appear different between DI water and the aqueous solution containing silica nanoparticles. The formation of smooth liquid columns is hypothesized to be a result the residence time is between emulsification and diffusion times, and is controlled by the injection speed. A few examples are shown indicating that it is possible to print liquid filaments.
Overall, the liquid-in-liquid printing results are interesting and reveal a new method for printing liquid structures. Although some of the stated claims are supported by the results, there are areas that need to be addressed and revised. The major concern is the classification that the microemulsion morphology is bicontinuous when silica nanoparticles are present. Once the authors address the minor comments below, the manuscript is publishable. Comments 1. How are the authors defining the bicontinuous morphology? In surfactant and diblock copolymer literature, the bicontinuous microemulsion structure is defined as having zero mean curvature and negative Gaussian curvature. The microemulsion phase in the SEM images look to have non-zero mean curvature. Therefore, it is highly unlikely that a bicontinuous morphology forms.
2. An alternative claim is that the silica nanoparticles preferentially organize that the aqueous/oil interface, creating a more elastic droplet interface that jams the system. Nanoparticles are known to be effective emulsifying agents. It is hard to determine from the SEM images, but it looks like the droplet diameter decreases with added silica nanoparticles. This would help confirm the emulsifying characteristics of the nanoparticles. 3. The rapid decrease in interfacial tension with the addition of nanoparticles suggests that they reside at the aqueous/oil interface. How does the stabilization time of the interfacial tension change of with silica nanoparticle wt%? 4. Page 7, Line 90: There may not be a change in the Span micelle morphology, but the optical and scanning electron microscopy images do not probe nanometer length scales that would be necessary to support the claim. Please revise. The comment is also made in the conclusion section. 5. There is a recent article showing that it is possible to print robust liquid filaments with internal phase nanostructures using both ionic and non-ionic surfactants (Macromol. Rapid Commun. 2021, 42, 2100445). Thus, the statement in the conclusion that 3D printing of micellar solutions is limited to ionic surfactants is not correct. The authors seemed to have missed the reference.
Reviewer #4 (Remarks to the Author): The comments are attached.
1. The discussion about underlying stabilization phenomena is not convincing and the arguments are not completely correct or well supported. For example, the term microemulsion are misused (e.g., figure 2 and line 166, line 83). In the colloids field, the microemulsion is referred to a thermodynamically-equilibrated phase that possess features in the order of only a few nanometers (such as micellar, inverse micellar) thus not possible to be captured in the micro-meter scale as in figures 2, S12, and S19. There are major differences in terms of stability and scales in emulsion and microelmulsion definitions that are not properly addressed throughout the paper. Furthermore, it is claimed that the in situ emulsification could be the underlying mechanism for stabilization of interface, however emulsified phases are often unstable with insufficient mechanical properties (such as water-in-oil droplets or oil-in-water emulsions), unless they have some internal nanostructures, that are not addressed here. It is also relevant to discuss the possible phase changes in emulsification or formation ouzu effects that are commonly used in the well-established area of nanoprecipitation. Also, it is not clear how the authors concluded that the micelle morphology remains unchanged in the presence of absence of nanoparticles (line 90) by looking on the SEM pictures (which has droplets of micrometer sizes), since the micelles (usually in the order of a few nanometers) cannot be captured in that image.
2. It is mentioned that the interfacial layer has elastic/viscoelastic properties in the paper. However, no characterization on interfacial properties has been performed to back it up. The authors are advised to perform necessary characterization such as interfacial rheology at the liquid-liquid interface or bulk rheology on isolated gel-like samples. Furthermore, the terms "elastic" and "viscoelastic" are used interchangeably (in the main text and in the supporting information). These two terms are not clearly the same. Use of interfacial and bulk rheology is needed to determine whether mechanical properties of interfacial materials suggest an elastic or viscoelastic behavior.
3. The manuscript shows interesting observations and exhaustive fluid dynamic data for a system of oil-surfactant-water (w or w/o nanoparticles). However, the advantages of the current system compared to other relevant systems are not well supported. The authors are advised to include convincing arguments concerning the novelty of their work. In doing so, the introduction should be more comprehensive covering current approaches in liquid-in-liquid printing (e.g., [1][2]4] and all various underlying mechanisms as well as a discussion section covering comparisons of their systems with others. The author mentioned a few comparisons somewhere else in the paper (jamming, phase transition, section 3.1) or SI (emulsion-based in table s2), but not discussed it thoroughly in the introduction and the discussion. Also, some of the advantages that claimed about the system are not convincing. For example, in table S2 (and throughout the main text) it is claimed that the system is capable of encapsulating cargos and method 1 and 2 are not. However, they have not performed any encapsulation test to support that. Also, the stability that claimed as the strength of their work (in table B2 and main text line 169) are not supported or at least not illustrated in Figure S18. Structures seem they have lost their shape (such as letter F, L). Another claim about the internal structure and porosity of the structure has not been tested (e.g., for a good porous example: [3]). It is not clear what length scale they are talking about for the internal structures. For example, they mentioned that the internal structure is not available for the method 1 in Table S2, however, the recent publication from that group supports the formation of internal nanostructure [4]. 1. Forth, J., Liu, X., Hasnain, J., Toor, A., Miszta, K., Shi, S., ... & Russell, T. P. (2018).
Reconfigurable printed liquids. Advanced Materials, 30(16), 1707603. 2. Lin, D., Liu, T., Yuan, Q., Yang, H., Ma, H., Shi, S., ... & Russell, T. P. (2020) 4. The description of initial oil phases and their interpretation are not clear. For example, 20 wt% span solution (in mineral oil) is claimed to be a micellar solution, however, since the continuous phase is an oil, one can expect the formation of reversed micelles and not micelles. Also, the rheological properties of that phase suggested a shear thinning behavior (Fig S1b), however, it is unlikely that the micellar phases alone can show non-Newtonian behavior. If possible, proper characterization (such as x-ray scattering) should to be used to support the internal structures of such phase. Authors need to integrate the structure and properties into understanding the underlying phenomena in the main text discussion.
5. The title should be more descriptive of what is presented in the paper, i.e., include the underlying mechanisms or process. Also, the title includes "printing" which is not investigated at all, as no 3D model was used for printing, rather all the liquid structures were created. Furthermore, it is not clear why the terms "spongy", "all-in-liquid" are used, for instance, spongy texture is not well illustrated in Fig 4f. The spongy structure actually is a well-defined nanostructure in colloids field and use of spongy liquid here could be misleading.
6. The authors assigned emulsification time t e and diffusion Damkoeler time t D based on some initial assumption on flow column morphology (as in Section 3.3 and fig 3a), which is questionable. Authors claimed that the time scale t R should be between emulsification time t e and diffusion Damkoeler time t D as the criterion for the formation of the liquid column. The argument to support that criterion is not clear. Also, there is almost no connection between the flow column morphological states observed in section (3.1) with printing application in section (3.3). It is advised to discuss what flow morphologies can be used for printing and what conditions should be considered.
Minor comments: 1. In line 29, the authors reported the average speed for injection, but there is no mention of how they calculate those values. 2. In line 138 specifically and throughout the manuscript in general, authors report that at an interfacial tension higher than 0.6 mN/m, the emulsification time is 0.2-0.5 s. From what we can see based on Figure 3b, we believe they meant 0.06, not 0.6 mN/m.

Reviewer #1 (Remarks to the Author):
The authors report a liquid-in-liquid printing process that creates a microemulsion or bicontinuous phase at the interface between the two liquids. There are a growing number of liquid-in-liquid printing processes. Unlike the authors' claim, it is entirely possible to print continuous threads and overcome the Plateau-Rayleigh instability. This cannot be refuted. See, for example, the work of Russell and co-workers on the use of nanoparticle-polymer surfactants, which enable that fabrication process in water-oil and water-water systems. Nevertheless, the creation of a mechanically robust microemulsion/bicontinuous phase at the interface during printing is interesting from a fundamental point of view.
What is perhaps unfortunate is the authors loose language and unsubstantiated claims in their discussion of some of the major outcomes. For example, the authors claim that they create "porous liquids" in their fabrication process, which does not appear supported by their data. In addition, the term "porous liquid" has already been taken, so to speak, by researchers elsewhere (Andy Cooper and others) who create porosity in liquids by using discrete and soluble/dispersible cages whose free volume excludes the solvent on the basis of size. There are many papers on this topic and therefore I am not sure why the authors have chosen this as a terminology for a result they have not provided evidence to support.
In addition, the authors have not explored much in the range of materials classes that might exhibit such behavior. In any sort of revision, if invited by the editor, an expansion in scope would need to be established. It would also be necessary to show with data what new or intriguing properties or functions are available to this class of liquid-in-liquid materials. The field of liquid-in-liquid printing has reached a level of maturity that variations on a theme are not likely to have impact unless the functions of those materials make use of the unique materials hierarchy. If such issues were addressed in a substantial revision, then it might be considered suitable for publication in Nature Communications. However, in its current state, only basic principles of fabrication have been established, and not the breadth of applicability or functionality enabled.
We appreciate the feedback provided by the reviewer, which has certainly helped us to improve the revised manuscript. We understand that the inappropriate use of the "porous liquid" terminology may have led to some sort of confusion regarding the novelty and application of our work, as we will discuss in this response letter. Next, one-by-one, we provide clarification regarding the concerns raised by the reviewer regarding the field of liquid-liquid printing and the generality of the present technique. We also provide details of the unique features related to the our development of structured liquids based on an interfacial mechanically robust microemulsion/bicontinuous phase at the interface.
Q.1.1 The authors report a liquid-in-liquid printing process that creates a microemulsion or bicontinuous phase at the interface between the two liquids. There are a growing number of liquidin-liquid printing processes. Unlike the authors' claim, it is entirely possible to print continuous threads and overcome the Plateau-Rayleigh instability. This cannot be refuted. See, for example, the work of Russell and co-workers on the use of nanoparticle-polymer surfactants, which enable that fabrication process in water-oil and water-water systems. Nevertheless, the creation of a mechanically robust microemulsion/bicontinuous phase at the interface during printing is interesting from a fundamental point of view. R. 1.1 The manuscript demonstrates the formation and application of spongy (stabilized with emulsion layers) liquid columns for utilization in printing liquid letters and creating liquid-fluid channels. We agree with the reviewer that there are growing numbers of recent publications (e.g., already in 2005 Subramanian et al. used particle-covered bubbles for arresting liquid-air interfaces in desired non-equilibrium shapes, which was later extended to liquid-liquid interfaces by Currently, there are two approaches for liquid-in-liquid printing: (i) nanoparticle-polymer jamming and (ii) change in lamella structures with cationic surfactant solutions and fatty acids (we discuss them in the revised introduction as below). Our contribution is introducing a new technique for printing all-in-liquid materials by forming an emulsion layer in-situ at the oil-water interface. We were pleased to read that the reviewer acknowledged that creating a robust emulsion layer at the interface is an interesting concept. We have developed a new methodology based on this concept for printing a liquid phase in the second immiscible liquid phase. We understand that our introduction in our first submitted manuscript may have brought the impression that we do not have a comprehensive understanding of this field, although we had a summary table mentioning the advantages/shortcoming of the previous works in the supplementary information. To resolve this issue, we added a detailed discussion on the history of liquid-in-liquid printing and the advances of recent works in the introduction (lines 6-35 in the revised manuscript) as: 4 Q1.2 What is perhaps unfortunate is the authors loose language and unsubstantiated claims in their discussion of some of the major outcomes. For example, the authors claim that they create "porous liquids" in their fabrication process, which does not appear supported by their data. In addition, the term "porous liquid" has already been taken, so to speak, by researchers elsewhere (Andy Cooper and others) who create porosity in liquids by using discrete and soluble/dispersible cages whose free volume excludes the solvent on the basis of size. There are many papers on this topic and therefore I am not sure why the authors have chosen this as a terminology for a result they have not provided evidence to support. R.1.2 We agree with the reviewer that the term "porous liquids" has been used loosely and have changed the terminology to "liquids with emulsified interfacial layers". This language can better represent the interfacial skin made of emulsion drops of silica nanoparticles packed in a continuous oil phase, analogous to the packed beads in synthetic porous media [Colligan et al. (2006), Gueven et al. (2017)]. Hence, the term porous is borrowed from porous media science, where porosity refers to the space between the compacted emulsion drops. We should draw the reviewer's attention to section IX in the SI (Figures S25-28), where the porosity of the interfacial layer, its generation and growth have been characterized. We have clarified our writing to make these parts of our contribution more clear and removed the "porous liquid" term throughout the manuscript. Q. 1.3 In addition, the authors have not explored much in the range of materials classes that might exhibit such behavior. R.1.3 Studying a wide of range of nanoparticle/surfactant/polymer composites is not feasible as changing materials may significantly alter the fluid dynamics and resulting structured liquids. It should be emphasized that the related published articles in the literature are also focused on one set of nanoparticle-polymer systems. In our work, we have conducted a detailed analysis on the time scales of various competing mechanisms in forming stabilized liquid columns. We have drawn general maps, based on dimensionless groups governing the flow and emulsification dynamics, where the favorable conditions to form structured liquids are distinguished. Such an analysis can be extended to other systems of nanoparticle-surfactant systems. Section X in SI provide a summary of the existing methodologies and the used material in each system, including the present work, and their advantageous and shortcomings (see Table S2). We have clarified our writing to make these parts of our contribution more clear.
The advantages of the developed technique can be summarized as: the printed spongy materials (i) are highly interconnected, (ii) can be spontaneously created in large volumes with minimal input energy, and (iii) provide high surface-to-volume ratios with the nanoparticles settled at the extended emulsion droplet surfaces, all unique characteristics of the present study. The printed features can be as small as a millimetre to a few centimeters with the micro-scale domain size. Emulsion drops, generated with the used materials in this work, are submicrometric, as they are packed into distinct layers with micrometer thicknesses. The interfacial skin in the present work is dynamic, meaning that the droplets in the outer layers are slowly detached and diffused into the surrounding phase while a fresh internal layer is generated. The rates of detachment and regeneration of droplets can be controlled or ceased by altering the fluids separated by the skin. As demonstrated in the manuscript, the tube can be readily contracted or expanded, like blood vessels, by the rate of the fluid pumped inside the tube. Moreover, the intensity of droplets can be controlled through the concentration of micelles. Although we have only shown the manual printing of a simple tube-like structure, more sophisticated structures can be printed using a programmable 3D printer.
In summary, liquid-in-liquid printing is an emerging field, with a historical growth of stabilizing liquid-fluid interfaces in desired non-equilibrium shapes from the work of We present a fundamental platform for the spontaneous structuring of emulsion drops in-situ, which is a missing link in the literature. The approach is built upon our recent report on the spontaneous formation of multiple emulsions, where a new interfacial material with unique layered microemulsion shells is generated. This has been accomplished by engineering the kinetic and hydrodynamic characteristics of the nanoparticle dispersion-micellar solution interfaces. Furthermore, the required criteria for fabricating the spongy all-in-liquid materials have been fully unravelled. In previous works, the liquid-liquid interface is stabilized by the nanoparticlesurfactant jamming or change in the micelle morphology. However, the stabilized liquid-liquid structures with the nanoparticle jamming suffer from the lack of internal structures. The recently published work by Honaryar et al. (2021) offers a pathway to create internal structures, however, the method requires photopolymerization and structures are not formed spontaneously. In a recent book published in "Royal Society of Chemistry, January 2020" entitled "Bijels: Bicontinuous Particle-stabilized Emulsions", a comprehensive review on different approaches for creating bicontinuous structures, their utilization in 3D printing systems, and their wide potential applications has been provided. The book concludes by noting the absence of an approach for creating spontaneous and droplet-based 3D printed all-in-liquid materials, i.e., self-derived compartmental emulsification without external energy, similar to what has been reported in the submitted manuscript. Table S2: Summary of liquid-in-liquid printing publications This paper presents a simple and novel approach to create stable liquid filaments of silica nanoparticle dispersions and propose to use them as inks to construct materials consisting a network of droplets. The phenomena is well characterized and demonstrated. The mechanism of forming the liquid filaments with the presence of silica particles is well explained. The strategy of creating liquid columns by in-situ emulsification is general and should be potentially beneficial to any applications that involve highly interconnected structures with high surface-to volume ratios. Therefore, I suggest to accept this manuscript after addressing the following minor comments.
We are pleased to read that the reviewer notes that the phenomena is well characterized and demonstrated. The mechanism of forming the liquid filaments with the presence of silica particles is well explained. The strategy of creating liquid columns by in-situ emulsification is general and should be potentially beneficial to any applications that involve highly interconnected structures with high surface-to volume ratios." Also, we are happy to read that the reviewer describes the approach as "novel" and recommends that the manuscript be accepted.
Q2.1. Figure 1d is not readable at all. I suggest to decompose it into several ones to make the point clearer. R2.1. As suggested by the reviewer we added 2D planes of Figure 1d to SI as Figure S12.
Q2.2. On page 7, line 84, the author state that "water droplets are Span micelles that are filled with water after being in contact with the aqueous phase". It would be nice if author can provide any evidence or reference to support this point. R2.2. The oil phase contains a high concentration of Span 80 surfactants; thus, reverse micelles are formed inside the oil phase. Before the contact with the aqueous phase, the size of the reverse micelle is 3-5 nm (measured with dynamic light scattering). We have clarified our writing to make these parts of our contribution more clear as: High magnification confocal images of the interconnected structures of oil and water that shows the presence of small droplets in the sample. Images are taken from a diluted region. Although there are many small sub-micrometer droplets within the sample, we cannot observe the submicrometric droplets due to the limitation in resolution. The submitted manuscript reports on liquid-in-liquid printing of filaments that are stabilized by emulsions that form when an aqueous solution containing silica nanoparticles come in contact with an oil phase containing Span 80 micelles. Four different flow regime morphologies are reported and include single droplets, bead-on-a-string, column, and connected. The transition from single droplets to the three other morphologies is a result of the addition of silica nanoparticles and increased injection speed. Optical and scanning electron microscopy images indicate that the emulsion morphologies appear different between DI water and the aqueous solution containing silica nanoparticles. The formation of smooth liquid columns is hypothesized to be a result the residence time is between emulsification and diffusion times, and is controlled by the injection speed. A few examples are shown indicating that it is possible to print liquid filaments.
Overall, the liquid-in-liquid printing results are interesting and reveal a new method for printing liquid structures. Although some of the stated claims are supported by the results, there are areas that need to be addressed and revised. The major concern is the classification that the microemulsion morphology is bicontinuous when silica nanoparticles are present. Once the authors address the minor comments below, the manuscript is publishable.
We are pleased to read that the reviewer wrote "the liquid-in-liquid printing results are interesting and reveal a new method for printing liquid structures" and recommends publication once minor comments are clarified. Comments Q3.1. How are the authors defining the bicontinuous morphology? In surfactant and diblock copolymer literature, the bicontinuous microemulsion structure is defined as having zero mean curvature and negative Gaussian curvature. The microemulsion phase in the SEM images look to have non-zero mean curvature. Therefore, it is highly unlikely that a bicontinuous morphology forms. R3.1. We consider the bicontinuous phase as an interconnected zone of oil and emulsion phase, depicted in Figures S15 and S20, not the emulsion zone by itself. In these images, the interfacial curvature between the oil and emulsion phase is close to zero (they form parallel zones), and similar structures have been reported for bijels [Huage et al. (2017)]. For clarification purposes, we have more details about "the bicontinuous phase" in the revised version as: Figure R1 a) interconnected structures of oil and emulsion phase. b) binarized image that clearly shows the oil phase is not in the forms of spherical droplets and it has arbitrary structures. Q3.
2. An alternative claim is that the silica nanoparticles preferentially organize that the aqueous/oil interface, creating a more elastic droplet interface that jams the system. Nanoparticles are known to be effective emulsifying agents. It is hard to determine from the SEM images, but it looks like the droplet diameter decreases with added silica nanoparticles. This would help confirm the emulsifying characteristics of the nanoparticles. R3.2. We thank the reviewer for their attention to details and the concise comment about reducing the droplet size upon adding the particles. We measured the emulsion droplet size from Cryo-SEM images and added the figure to supporting document as Figure S16. We also added a comment in the manuscript in this regard as: Q3.3. The rapid decrease in interfacial tension with the addition of nanoparticles suggests that they reside at the aqueous/oil interface. How does the stabilization time of the interfacial tension change of with silica nanoparticle wt.%? R3.3. We added the dynamic interfacial tension data for all the tested concentrations in Figure S2.
In the presence of particles, the interfacial tension values are smaller and the reduction in interfacial tension is faster and reach the equilibrium state in less than a few minutes. The stabilization time is a decreasing function of silica nanoparticle concentration.
Q3.4. Page 7, Line 90: There may not be a change in the Span micelle morphology, but the optical and scanning electron microscopy images do not probe nanometer length scales that would be necessary to support the claim. Please revise. The comment is also made in the conclusion section. R3.4. We agree with the reviewer's comment that we may have lost some details in the change of the morphology on a smaller scale. Thus, as suggested, we remove this claim.
Q3.5. There is a recent article showing that it is possible to print robust liquid filaments with internal phase nanostructures using both ionic and non-ionic surfactants (Macromol. Rapid Commun. 2021, 42, 2100445). Thus, the statement in the conclusion that 3D printing of micellar solutions is limited to ionic surfactants is not correct. The authors seemed to have missed the reference. R3.5. Thank you for pointing out this reference. We discussed this paper in the revised manuscript and adjusted the related sections accordingly as:

Reviewer #4 (Remarks to the Author):
In this manuscript, the authors study the morphological states and flow instabilities of ternary/quaternary mixtures of mineral oil-Span-water (w or w/o Silica nanoparticles) with some applications in printing. There are some concerns regarding the validity of the proposed mechanism and the significance of the work. Also, the connection between fluid dynamic results and direct applicability of them to printing is not discussed. I have following specific comments.
Major comments: Q4.1. The discussion about underlying stabilization phenomena is not convincing and the arguments are not completely correct or well supported. For example, the term microemulsion are misused (e.g., figure 2 and line 166, line 83). In the colloids field, the microemulsion is referred to a thermodynamically-equilibrated phase that possess features in the order of only a few nanometers (such as micellar, inverse micellar) thus not possible to be captured in the micro-meter scale as in figures 2, S12, and S19. There are major differences in terms of stability and scales in emulsion and microelmulsion definitions that are not properly addressed throughout the paper. Furthermore, it is claimed that the in-situ emulsification could be the underlying mechanism for stabilization of interface, however emulsified phases are often unstable with insufficient mechanical properties (such as water-in-oil droplets or oil-in-water emulsions), unless they have some internal nanostructures, that are not addressed here. It is also relevant to discuss the possible phase changes in emulsification or formation ouzu effects that are commonly used in the well-established area of nanoprecipitation. Also, it is not clear how the authors concluded that the micelle morphology remains unchanged in the presence of absence of nanoparticles (line 90) by looking on the SEM pictures (which has droplets of micrometer sizes), since the micelles (usually in the order of a few nanometers) cannot be captured in that image. R4.1. We thank reviewer 4 for the comprehensive and constructive comments that helped us to clarify the manuscript, including small issues of language in the literature that are noted above. We addressed each concern raised by the reviewer as follow: i) For example, the term microemulsion are misused (e.g., figure 2 and line 166, line 83). In the colloids field, the microemulsion is referred to a thermodynamically-equilibrated phase that possess features in the order of only a few nanometers (such as micellar, inverse micellar) thus not possible to be captured in the micro-meter scale as in figures 2, S12, and S19. There are major differences in terms of stability and scales in emulsion and microelmulsion definitions that are not properly addressed throughout the paper. Microemulsions are generally referred to as systems where the dispersed domain size is less than 10 nm; thus, they are transparent [Lopez et al. (2002)]. We understand that, although in our case emulsions are formed spontaneously, they are not microemulsions. We changed the microemulsion terminology to emulsion throughout the manuscript (though the size scale of the emulsions we observe are micrometers), as suggested by the reviewer.

ii)
Furthermore, it is claimed that the in-situ emulsification could be the underlying mechanism for stabilization of interface, however emulsified phases are often unstable with insufficient mechanical properties (such as water-in-oil droplets or oil-in-water emulsions), unless they have some internal nanostructures, that are not addressed here. Mixing two immiscible phases with a shaker/homogenizer/sonicator generates a kinematically stable dispersion that undergoes destabilization over time. However, in our system, emulsions are generated spontaneously at the interface, and they are thermodynamically stable as they are formed without input energy. Following this comment, we added a discussion on emulsification mechanisms in our systems as: A concentrated layer of emulsion at the interface is sufficient to form the liquid filament as the liquid filaments remain intact under injection/re-injection cycles, as shown in Figure 4.

iii)
It is also relevant to discuss the possible phase changes in emulsification or formation ouzu effects that are commonly used in the well-established area of nanoprecipitation. We added a discussion on the formation of bicontinuous emulsions with phase change. For the ouzo effect, the presence of a third phase with mutual solubility with two other liquid phases is required. For example, the emulsification by the ouzo effect can occur when a mixture of oil and ethanol is placed in contact with water. As alcohol diffuses from the oil into the water, it carries some oil into the water; by further diffusion of alcohol, the oil phase in water forms small droplets. In our case, we do not have a component with mutual solubility; thus, we do not have the ouzo effect.

iv)
Also, it is not clear how the authors concluded that the micelle morphology remains unchanged in the presence of absence of nanoparticles (line 90) by looking on the SEM pictures (which has droplets of micrometer sizes), since the micelles (usually in the order of a few nanometers) cannot be captured in that image. We drew this conclusion from the SEM images. To clarify, we meant that there was not any transition to lamella structures. However, as there might be some changes in morphology at the nm scale, which we cannot resolve, we edited the statement and removed the comment about the micelle morphology. Q4.2. It is mentioned that the interfacial layer has elastic/viscoelastic properties in the paper. However, no characterization on interfacial properties has been performed to back it up. The authors are advised to perform necessary characterization such as interfacial rheology at the liquidliquid interface or bulk rheology on isolated gel-like samples. Furthermore, the terms "elastic" and "viscoelastic" are used interchangeably (in the main text and in the supporting information). These two terms are not clearly the same. Use of interfacial and bulk rheology is needed to determine whether mechanical properties of interfacial materials suggest an elastic or viscoelastic behavior. R4.2. Following the reviewer's comment, we performed rheological characterization to determine the mechanical properties of the interfacial layer. We used an Anton Paar-DHR-2 Rheometer using cone and plate with a cone diameter of 20 mm and angle of 2 deg. First, the aqueous and oil phases were placed in contact for one week to generate enough samples for the measurement. Then, the sample was collected from the interface, and the rheology experiment was conducted for the generated bulk material at the interface and results are presented in Figure S19. According to the measurements, the samples show both viscous and elastic properties; thus, they are viscoelastic, and we use this terminology throughout the manuscript. Q4.3. The manuscript shows interesting observations and exhaustive fluid dynamic data for a system of oil-surfactant-water (w or w/o nanoparticles). However, the advantages of the current system compared to other relevant systems are not well supported. The authors are advised to include convincing arguments concerning the novelty of their work. In doing so, the introduction should be more comprehensive covering current approaches in liquid-in-liquid printing (e.g., [1][2]4] and all various underlying mechanisms as well as a discussion section covering comparisons of their systems with others. The author mentioned a few comparisons somewhere else in the paper (jamming, phase transition, section 3.1) or SI (emulsion-based in table s2), but not discussed it thoroughly in the introduction and the discussion. Also, some of the advantages that claimed about the system are not convincing. For example, in table S2 (and throughout the main text) it is claimed that the system is capable of encapsulating cargos and method 1 and 2 are not. However, they have not performed any encapsulation test to support that. Also, the stability that claimed as the strength of their work (in table B2 and main text line 169) are not supported or at least not illustrated in Figure S18. Structures seem they have lost their shape (such as letter F, L). Another claim about the internal structure and porosity of the structure has 2 not been tested (e.g., for a good porous example: [3]). It is not clear what length scale they are talking about for the internal structures. For example, they mentioned that the internal structure is not available for the method 1 in Table S2, however, the recent publication from that group supports the formation of internal nanostructure: ii) For example, in table S2 (and throughout the main text) it is claimed that the system is capable of encapsulating cargos and method 1 and 2 are not. However, they have not performed any encapsulation test to support that. We made this claim as we have reverse micelles in the system, and they are capable of encapsulating aqueous phases and hydrophilic components. We added high-resolution confocal microscopy images in Figure S17 showing the nanoparticle dispersion inside the emulsion droplets. This shows the encapsulation capability as a hydrophilic phase (silica dispersion) is encapsulated inside a hydrophobic phase (oil). In the image, the green color shows the oil phase, and the red color shows the nanoparticle dispersion tagged with Rod-amine B dye.
iii) Another claim about the internal structure and porosity of the structure has 2 not been tested (e.g., for a good porous example: [3]). We analyzed the porosity of the 3D printed structures in Figures S25-28. The porosity increases sharpy within first 20 minutes and it reaches a plateau of ~60%. iv) Stability Without polymerization, previous works reported stability in terms of a few hours. Our printed liquid letters remain stable up to two months. Although, they lost some of their sharp edges over time, they have not dissolved completely. The stability of the printed letters is quantified in Figure  4b.
v) It is not clear what length scale they are talking about for the internal structures. For example, they mentioned that the internal structure is not available for the method 1 in Table S2, however, the recent publication from that group supports the formation of internal nanostructure: We have submicrometric domain internal structures made of emulsion droplets as illustrated in SEM images and their quantification (Figure2 and Figures S15-16). We discussed the publications on the internal structure, suggested by the reviewer, in the introduction. Q4.4. The description of initial oil phases and their interpretation are not clear. For example, 20 wt% span solution (in mineral oil) is claimed to be a micellar solution, however, since the continuous phase is an oil, one can expect the formation of reversed micelles and not micelles. Also, the rheological properties of that phase suggested a shear thinning behavior (Fig S1b), however, it is unlikely that the micellar phases alone can show non-Newtonian behavior. If possible, proper characterization (such as x-ray scattering) should to be used to support the internal structures of such phase. Authors need to integrate the structure and properties into understanding the underlying phenomena in the main text discussion. R4.4. In this manuscript, by micelle, we meant reverse micelles as the micelles are inside the oil phase. To avoid confusion, we changed micelle to reverse micelle throughout the manuscript. To address the comment regarding the micellar (reverse micellar) solution's shear-thinning behaviour, we measured the mineral oil's viscosity (without any surfactant in the system). The rheology plot is similar to the one with surfactant micelles. We checked the original papers on the shear viscosity of the mineral oil. Within the range of measurements (3.3-20 1/s) [Iaona (2011)], the plot shows a Newtonian behaviour as is the case for our measurement. However, the viscosity has not been reported at a lower shear rate. Thus, we believe the non-Newtonian behaviour of the micellar solutions at shear rates lower than 1 (1/s) is related to the properties of the mineral oil, and it is not the effect of micelles.
We cannot capture the shape of the micelles in the absence of water as they are around 5 nm as confirmed through dynamic light scattering measurements. However, the emulsion drops in the oil phase are spherical. To characterize the structure of the reversed micelles using x-ray scattering in a liquid media, we need a SAXS instrument with the pinhole of 12IDB. Unfortunately, we currently do not have access to this instrument. Figure R2: Shear viscosity of the mineral oil sample (without any surfactant). Q4.5. The title should be more descriptive of what is presented in the paper, i.e., include the underlying mechanisms or process. Also, the title includes "printing" which is not investigated at all, as no 3D model was used for printing, rather all the liquid structures were created. Furthermore, it is not clear why the terms "spongy", "all-in-liquid" are used, for instance, spongy texture is not well illustrated in Fig 4f. The spongy structure actually is a well-defined nanostructure in colloids field and use of spongy liquid here could be misleading. R4.5. Thank you for this suggestion. We changed the title to "Spongy all-in-liquid materials: Attenuating Rayleigh-Plateau instability by in-situ formation of emulsions at oil-water interfaces" to better describe the novelty of the work. Spongy is used to represent the texture of interfacial layer which is made of emulsions. Furthermore, the emulsification can proceed and the entire printed texture can turn to emulsion. We investigated the printing in Figure 4 for liquid letters and liquid-fluidic channel. The spongy texture is quantified by the term of porosity in Figure S28.
Q4.6. The authors assigned emulsification time te and diffusion Damkoeler time tD based on some initial assumption on flow column morphology (as in Section 3.3 and fig 3a), which is questionable. Authors claimed that the time scale tR should be between emulsification time te and diffusion Damkoeler time tD as the criterion for the formation of the liquid column. The argument to support that criterion is not clear. Also, there is almost no connection between the flow column morphological states observed in section (3.1) with printing application in section (3.3). It is advised to discuss what flow morphologies can be used for printing and what conditions should be considered. R4.6. Flow morphologies in Figure 1 and Figures S3-6 are identified based on the Fourier transfer analysis as described in supporting document section III. The Fourier analysis forms the basis for the quantified criteria in distinguishing the flow patterns as opposed to visual examination of images. Thus, the time scales, namely and are found experimentally and based on Fourier analysis to identify the transition from BOAS to column and column to connected flow regimes, respectively. Inspection of these time scales and their relations to the fluid properties reveals that and re directly proportional to the equilibrium interfacial tension ( ) and the micellar solution viscosity ( ), respectively. Through dimensional analysis and with the assumption that the equilibrium interfacial tension and the micellar solution viscosity are the dominant factors in emulsification and diffusion time scales, we could develop two correlations describing the time scales as = (6 ℓ 2 / ) and = ( 2 / ) ; where a, ℓ, kB, and T are, respectively, the micelle diameter, diffusion length scale, Boltzmann constant, and temperature (see supporting information section VII.1-2 and the main text section 3.3). We used these correlations to develop the generalized map describing the flow morphologies and the desired column regime as depicted in Figures 3c-d. Following this comment, we added a section in the manuscript on the related mechanisms of filament stability and clarify the emulsification and diffusion time scales, and also provided a discussion on the effect of the third phase (emulsion) formation at the oil-water interface on the attenuation or promotion of Rayleigh-Plateau instability. The added discussion is: Revised section in the manuscript: